Los Leones bagged the game-winner in the 63rd minute. Markel Susaeta slotted home into the lower left of the net after intercepting Marc Bertrán's pass and finishing the move with a right-footed shot.
Osasuna are in 15th place, just three points above the relegation zone with a record of seven wins, 12 losses and seven draws after 26 games.
Athletic Bilbao have a record of eight wins, 13 losses and five draws this season, which puts them in 14th place, only four points above the relegation zone.
Los Rojiblancos gained a win even though Osasuna managed to take more shots on goal during the course of play.
Rojillos fired in seven shots, six of which were accurate, compared to Athletic Bilbao's shots on goal total of two.
Gorka Iraizoz stopped six shots.
Ander had two shots for Athletic Bilbao, more than any of his team-mates.
The combination of Andoni Iraola to Susaeta was the most common during the match, as the two players connected 17 times.
Osasuna controlled possession for 54.7% of the contest thanks to strong open field play.
Osasuna won 80 duels and 32 headers, which led to seven shots, six of which were accurate.
The seven attempts by Emiliano Armenteros were the most of any Osasuna player.
Lolo's 14 passes to Armenteros were the most between any Osasuna team-mates during the match.